Artificial night lighting affects dawn song, extra-pair siring success, and lay date in songbirds

Associated with a continued global increase in urbanization[1], anthropogenic light pollution is an important problem[2]. However, our understanding of the ecological conse-quences of light pollution is limited [2–4]. We investigatedeffects of artificial night lighting on dawn song in fivecommon forest-breeding songbirds. In four species, malesnear street lights started singing significantly earlier atdawn than males elsewhere in the forest, and this effectwas stronger in naturally earlier-singing species. We com-pared reproductive behavior of blue tits breeding in edgeterritories with and without street lights to that of blue titsbreeding in central territories over a 7 year period. Underthe influence of street lights, females started egg laying onaverage 1.5 days earlier. Males occupying edge territorieswith street lights were twice as successful in obtainingextra-pair mates than their close neighbors or than malesoccupying central forest territories. Artificial night lightingaffected both age classes but had a stronger effect on year-ling males. Our findings indicate that light pollution hassubstantial effects on the timing of reproductive behaviorand on individual mating patterns. It may have importantevolutionary consequences by changing the informationembedded in previously reliable quality-indicator traits